PRE 
the ecliptic, which is 25,791 years ; and in 
that length of time, the north pole of the 
earth’s axis produced, describes the circle 
ABCDA in the starry heavens, round the 
pole of the ecliptic, which keeps immove- 
able in the centie of that circle. The 
earth’s axis being now 23" 28' inclined to 
the axis of the ecliptic, the circle ABCDA, 
described by the north pole of the earth’s 
axis produced to A, is 46" 56' in diameter, 
or double the inclination of the earth’s axis. 
In consequence of this, the point A, w’hich 
is at present the north pole of the heavens, 
and near to a star of the 2d magnitude in 
the end of the Little Bear’s tail, must be 
deserted by the earth’s axis ; which moving 
backwards one degree every 71f years 
nearly, will be directed towards the sb r or 
point is in 6447| years hence ; and in dou- 
ble of that time, or 12,895t years, it will be 
directed towards the star or point C ; which 
will then be the north pole of the heavens, 
although it is at present 8i degrees south 
of the zenith of London L. The present 
position of the equator EOQ will then be 
changed into eOq, the tropic of cancer 
TggZ into Vtog, and the tropic of capri- 
corn VTVj’ into fVJZ ; as is evident by the 
figure. And the sun, in the same part of 
the heavens where he is now over the earth- 
ly tropic of capricorn, and makes the short- 
est days and longest nights in the north- 
ern hemisphere, will then be over the earth- 
ly tropic of cancer, and make the days 
longest and nights shortest. So that it 
will require 12,895t years yet more, or 
from that time, to bring the north pole N 
quite rorind,. so as to be directed towards 
that point of the heavens which is vertical 
to it at present. And then, and not till 
then, the same stars which at present de- 
scribe the equator, tropics, and polar cir- 
cles, &c. by the earth’s diurnal motion, 
will describe them ever again. 
From this shifting of the equinoctial points, 
and with them all the signs of the ecliptic, 
it follows, that those starsj which in the in- 
fancy of astronomy were in Aries, are now 
found in Taurus ; those of Taurus in Ge- 
mini, &c. Hence likewise it is, that the 
stars which rose or set at any particular 
season of the year, in the times of Hesiod, 
Eudoxus, Virgil, Pliny, &c. by no means 
answer at this time to their descriptions. 
As to the physical cause of the precession 
of the equinoxes. Sir Isaac Newton demon- 
strates, that it arises from the broad or flat 
spheroidal figure of the earth ; which itself 
arises from the earth’s rotation about its 
PRE 
axis : for as more matter has thus been ac. 
cumulated all rdund the equatorial parts 
than any where else on the earth, the sun 
and moon, when on either side of the equator, 
by attracting this redundant matter^ bring 
the equator sooner under them, in every 
return towards it, than if there tvas no such 
accumulation. 
Sir Isaac Newton, in determining the 
quantity of the annual precession from the 
theory of gravity, on supposition that the 
equatorial diameter of the earth is to the 
polar diameter, as 230 to 229^ finds the sun’s 
action sufficient to produce a precession of 
9''i only ; and collecting from the tides the 
proportion between the sun’s force and fhe 
moon’s, to be as 1 to 4|, he settles the mean 
precession residting from their joint actions 
at 50" ; which is nearly the same as it has 
since been found by the best observations. 
PRECIjE, in botany, the name of the 
twenty-first order in Linnaeus’s Fragments 
of a Natural Method; containing the prim- 
rose, and a few other plants which agree 
with it in habit and structure. 
PRECIPITATE, in chemistry, is any 
matter or substance, which having been 
dissolved in a fluid, falls to the bottom of 
the vessel on the addition of some other 
substance, capable of producing a decom- 
position of the compound. The term is 
generally applied when the separation takes 
place in a flocculent or pulverulent form, in 
opposition to crystallization, which implies 
a like separation in an angular form. But 
chemists call a mass of crystals a precipi- 
tate, when they subside so suddenly, that 
their proper crystalline shape cannot be 
distinguished by the naked eye, as in the 
instance of Glauber’s salt, when separated 
from its watery solution by mixing with it a 
portion of alcohol. 
PRECIPITATION, that process by 
which bodies dissolved, mixed, or suspend- 
ed in a fluid, are separated from the fluid, 
and made to gravitate to the bottom of the 
vessel : this is one of the great operations in 
chemistry, 'and is opposed to that of solu- 
tion. In truth the chief operations in the 
laboratory may be resolved into solution 
and precipitation. When a base is employ- 
ed to precipitate a soluble acid, the sub- 
stance thrown down is always a compound, 
consisting of the acid united to the base 
employed. In this case the acid is some- 
times completely separated, and sometimes 
not, according to the energy of the base 
employed, and the degree of insolubility of 
the salt formed. The same explanation 
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